Joël Gaubicher

Li / β ‐ VOPO 4: A New 4 V System for Lithium Batteries

Lithium intercalation into {beta}-VOPO{sub 4} has been performed, both chemically and electrochemically, with X-ray diffraction characterization of the resulting phases at various levels of intercalation-deintercalation. It is shown that intercalation occurs through a first-order transition at 3.98 V vs. Li{sup +}/Li{sup 0}, which leads to a {beta}-LiVOPO{sub 4}-like structure but with lithium ordering which doubles the a and b unit cell parameters. Starting from chemically intercalated {beta}-Li{sub 0.92}VOPO{sub 4}, the reversible process involves 0.55 electron per transition metal, i.e., 90 mAh/g, at a C/50 rate which owing to the 3.95 V potential plateau, gives an attractive specific energy of 355 Wh/kg. Fundamentally, it is shown herein that the redox energy of V{sup 5+}N{sup 4+} couple in an octahedral coordination involving (V = O){sup m+} unit is included between those observed in oxides and in M{sub 2}(PO{sub 4}){sub 3} compounds with (PO{sub 4}){sup 3{minus}} oxo-anions.

Elucidation of the Na2/3FePO4 and Li2/3FePO4 Intermediate Superstructure Revealing a Pseudouniform Ordering in 2D

Based on TEM, synchrotron X-ray diffraction, DFT calculations, and Mössbauer spectroscopy, a unified understanding of the Na and Li intercalation process in FePO4 is proposed. The key to this lies in solving the highly sought-after intermediate A(2/3)FePO4 (A = Na, Li) superstructures that are characterized by alkali ions as well as Fe(II)/Fe(III) charge orderings in a monoclinic three-fold supercell. Formation energies and electrochemical potential calculations confirm that Na(2/3)FePO4 and Li(2/3)FePO4 are stable and metastable, respectively, and that they yield insertion potentials in fair agreement with experimental values. The 2/3 Na(Li) and 1/3 vacancy sublattice of the intermediate phases forms a dense (101)(Pnma) plane in which the atom/vacancy ordering is very similar to that predicted for the most uniform distribution of 1/3 of vacancies in a 2D square lattice. Structural analysis strongly suggests that the key role of this dense plane is to constrain the intercalation in the diffusion channels to operate by cooperative filling of (bc)(Pnma). From a practical point of view, this generalized mechanism highlights the fact that an interesting strategy for obtaining high-rate FePO4 materials would consist in designing grains with an enhanced (101) surface area, thereby offering potential for substantial improvements with respect to the performance of rechargeable Li and Na batteries.

In situ redox functionalization of composite electrodes for high power-high energy electrochemical storage systems via a non-covalent approach

The growing demand for new global resources of clean and sustainable energy emerges as the greatest challenge in todays society. For numerous applications such as hybrid vehicles, electrochemical storage systems simultaneously require high energy and high power. For this reason, intensive researches focus on proposing alternative devices to conventional Li battery and supercapacitors. Here, we report a proof of concept based on non-covalent redox functionalization of composite electrodes that may occur either during the calendar life or during the device functioning. The active material, a multi-redox pyrene derivative, is initially contained in the electrolyte. No additional benchmarking step is therefore required, and it can, in principle, be readily applied to any type of composite electrode (supercapacitors, battery, semi-solid flow celletc.). Accordingly, a practical carbon fiber electrode that is 10 mg cm−2 loaded can deliver up to 130 kW kgelectrode−1 and 130 Wh kgelectrode−1 with negligible capacity loss over the first 60 000 charge/discharge cycles.

Lowering interfacial chemical reactivity of oxide materials for lithium batteries. A molecular grafting approach

This paper proposes a molecular grafting approach as a way to modify the interfacial chemical reactivity of oxide materials, which is detrimental to their long-term energy storage properties. The present study demonstrates that diazonium chemistry offers an efficient path to graft molecules at the surface of a powder oxide material. Indeed, surface derivatization of Li1.1V3O8 nanograins was accomplished by in situ electrografting of a diazonium salt during Li-ion intercalation. The results show that aryl molecules are strongly bonded to the surface forming an organic multilayer the thickness of which can be modulated. Based on TEM, XPS and electrochemical probing of the surface reactivity, the results demonstrate the interest of the proposed surface modifications as a way to tailor both electrochemical and chemical reactivities of oxide electrode materials. Interestingly, charge transfer at the surface of the material is not impeded, while electrolyte decomposition is inhibited. It is anticipated that molecular derivatization of electrode surfaces is a new research direction which will be developed in the field of battery science in the near future, in order to prevent targeted side reactions occurring at different steps of battery manufacturing and use, such as storage, electrode processing, simple contact with electrolyte, and cycling.

Mechanism of Silicon Electrode Aging upon Cycling in Full Lithium-Ion Batteries.

Understanding the aging mechanism of silicon-based negative electrodes for lithium-ion batteries upon cycling is essential to solve the problem of low coulombic efficiency and capacity fading and further to implement this new high-capacity material in commercial cells. Nevertheless, such studies have so far focused on half cells in which silicon is cycled versus an infinite reservoir of lithium. In the present work, the aging mechanism of silicon-based electrodes is studied upon cycling in a full Li-ion cell configuration with LiCoO2 as the positive electrode. Postmortem analyses of both electrodes clearly indicate that neither one of them contains lithium and that no discernible degradation results from the cycling. The aging mechanism can be explained by the reduction of solvent molecules. Electrons extracted from the positive electrode are responsible for an internal imbalance in the cell, which results in progressive slippage of the electrodes and reduces the compositional range of cyclable lithium ions for both electrodes.

Synergistic effect in carbon coated LiFePO4 for high yield spontaneous grafting of diazonium salt. Structural examination at the grain agglomerate scale.

Molecular grafting of p-nitrobenzene diazonium salt at the surface of (Li)FePO4-based materials was thoroughly investigated. The grafting yields obtained by FTIR, XPS, and elemental analysis for core shell LiFePO4-C are found to be much higher than the sum of those associated with either the LiFePO4 core or the carbon shell alone, thereby revealing a synergistic effect. Electrochemical, XRD, and EELS experiments demonstrate that this effect stems from the strong participation of the LiFePO4 core that delivers large amounts of electrons to the carbon substrate at a constant energy, above the Fermi level of the diazonium salt. Correspondingly large multilayer anisotropic structures that are associated with outstanding grafting yields could be observed from TEM experiments. Results therefore constitute strong evidence of a grafting mechanism where homolytic cleavage of the N2(+) species occurs together with the formation and grafting of radical nitro-aryl intermediates. Although the oxidation and concomitant Li deintercalation of LiFePO4 grains constitute the main driving force of the functionalization reaction, EFTEM EELS mapping shows a striking lack of spatial correlation between grafted grains and oxidized ones.

Formation of Li1 + n V3O8 ∕ β-Li1 ∕ 3V2O5 ∕ C Nanocomposites by Carboreduction and the Resulting Improvement in Li Capacity Retention

The formation of the xerogel precursor of Li 1 + α V 3 O 8 within a suspension of carbon black allows the synthesis of Li 1 + α + x V 3 O 8 /β-Li 1 / 3 V 2 O 5 /C nanocomposites upon heating under argon at 350°C for a few minutes. The intimate contact of the active material with carbon particles ensures a limitation of the particle growth, a reduction-insertion of the pristine Li 1 + α V 3 O 8 compound along with the formation of β-Li 1 / 3 V 2 O 5 , and an efficient electronic transport to the active materials. The electrochemical performance of these nanocomposites is significantly better in terms of both initial capacity and capacity retention upon cycling than that of standard Li 1 + α V 3 O 8 .

Toward fully organic rechargeable charge storage devices based on carbon electrodes grafted with redox molecules

Activated carbon powders modified with naphthalimide and 2,2,6,6-tetramethylpiperidine-N-oxyl were assembled into a hybrid electrochemical capacitor containing an organic electrolyte. The fully organic rechargeable system demonstrated an increase in specific capacitance up to 51%, an extended operating voltage of 2.9 V in propylene carbonate, compared to 1.9 V for the unmodified system, and a power 2.5 times higher.

Modification of activated carbons based on diazonium ionsin situ produced from aminobenzene organic acid without addition of other acid

Activated carbon products modified with a benzene sulfonic acid group were prepared based on the spontaneous reduction of diazonium salts in situ generated in water without addition of an external acid. The diazotization reaction assisted by the organic acid substituent, produced at once amine, diazonium and triazene functionalities that maximize the grafting yield by a chemical cooperation effect.

Crystal structure of the end product of electrochemical lithium intercalation in V2(SO4)3

The crystal structure of Li2V2(SO4)3 obtained by electrochemical intercalation of lithium into V2(SO4)3 has been solved using high resolution synchrotron X-ray powder diffraction. Surprisingly, in light of its ‘soft chemistry’ synthesis, the structure of Li2V2(SO4)3 is derived from that of the starting material via rearrangement of strong V–O bonds, leading to a mixed corner sharing/edge sharing arrangement among the VO6 octahedra and SO4 tetrahedra. Results of in situ electrochemical Li intercalation/synchrotron powder diffraction studies, carried out in working batteries and with a time resolution of the order of minutes, indicate that the rearrangement is reversible, at least during the first cycle.